1. Field of the Invention
The present invention relates to an integrated degenerative amplifier, and more particularly to a current amplifier with at least two amplifier stages and a degenerative feedback branch guided from the amplifier output to the amplifier input.
2. Description of the Prior Art
Analog amplifiers, which are used for amplifying detector output signals, must have degenerative coupling to achieve stability. Photo-detectors, for instance, function as a current source so that the amplifier must be constructed as a current amplifier.
If such amplifiers are constructed as integrated circuits, it would be preferable to embody the circuit without resistors, and particularly without degenerative feedback resistors, since resistors with a high resistance value are difficult to produce in integrated circuit techniques. Integrated resistors require a relatively large surface in the integrated circuit so that no great packing densities can be obtained in the integrated circuit. Since the magnitude of the resistance value which can be realized in integrated circuits is limited, only average current sensitivities can be obtained in fully integrated degenerative amplifiers.
However, problems also occur when external degenerative feedback resistors are used in highly sensitive current amplifiers. In the case of photodiodes in automatic exposure systems for photographic cameras, for instance, the photocurrent in the most sensitive range amounts to a few pico-amperes. The degenerative feedback resistance should then be greater than 10.sup.10 Ohms. Such resistances, however, have dimensions which are too large for automatic exposure systems and are also too expensive.